Role of the LuxR family transcriptional regulator Lpg2524 in the survival of Legionella pneumophila in water.

The water-borne Gram-negative bacterium Legionella pneumophila (Lp) is the causative agent of Legionnaires' disease. Lp is typically transmitted to humans from water systems, where it grows inside amoebae. Survival of Lp in water is central to its transmission to humans. A transcriptomic study previously identified many genes induced by Lp in water. One such gene, lpg2524, encodes a putative LuxR family transcriptional regulator. It was hypothesized that this gene could be involved in the survival of Lp in water. Deletion of lpg2524 does not affect the growth of Lp in rich medium, in the amoeba Acanthamoeba castellanii, or in human macrophage-like THP-1 cells, showing that Lpg2524 is not required for growth in vitro and in vivo. Nevertheless, deletion of lpg2524 results in a faster colony-forming unit (CFU) reduction in an artificial freshwater medium, Fraquil, indicating that Lpg2524 is important for Lp to survive in water. Overexpression of Lpg2524 also results in a survival defect, suggesting that a precise level of this transcriptional regulator is essential for its function. However, our result shows that Lpg2524 is dispensable for survival in water when Lp is at a high cell density (10(9) CFU/mL), suggesting that its regulon is regulated by another regulator activated at high cell density.